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Mitchell, Edward

Nom
Mitchell, Edward
Affiliation principale
Site web
Fonction
Professeur ordinaire
Email
edward.mitchell@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/357
_
0000-0003-0358-506X

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  • Publication
    Accès libre
    A molecular approach to microeukaryotic diversity, ecology and biogeography associated with Sphagnum mosses
    (2017)
    Singer, David,
    ;
    Lara, Enrique 
    ;
    Mitchell, Edward 
    Malgré le fait que les micro-eucaryotes composent la majeure partie de la biodiversité terrestre et jouent de nombreux rôles essentiels dans le maintien des écosystèmes, la connaissance de leur diversité, de leur écologie ainsi que de leurs aires de répartition reste très lacunaire. Dans ce sens, les objectifs de cette thèse sont 1) d’accroître la connaissance de la diversité des micro-eucaryotes 2) de caractériser les préférences écologiques et de déterminer quelles sont les principales variables qui influencent la composition des communautés et enfin 3) de comprendre les règles qui dirigent les communautés à l’échelle locale et globale. Pour atteindre ces objectifs, un milieu spécifique a été sélectionné : la "sphagnosphère", celui-ci désigne l’eau interstitielle sous l’influence des mousses de sphaignes (Sphagnum). Cet environnement est un excellent modèle en biologie car il se caractérise par une faible teneur en éléments nutritifs, un faible pH, des quantités élevées d’acides organiques et une grande stabilité dans le temps.
    Nous avons d’abord exploré la diversité de deux groupes de protistes vivant dans les sphaignes. Le premier groupe est le genre Nebela (Arcellinida, Hyalospheniidae), un groupe d’amibes à thèque composé d’espèces étroitement apparentées. Nous avons décrit formellement la plus abondante et l’avons nommée Nebela gimllii en raison de la taille de sa thèque. Les différents profils de communautés ont révélé que les espèces ne sont pas distribuées de manière aléatoire dans les tourbières. Au contraire, nous avons observé un fort groupement phylogénétique dans les zones oligotrophes, ce qui suggère que les teneurs faibles en azote exercent une forte pression environnementale. Nous avons également étudié la diversité moléculaire du clade d’Oomycota. Ce sont des stramenopiles qui se composent de nombreux parasites d’animaux, de champignons et de végétaux, ainsi que d’espèces saprotrophes. Nous avons révélé une grande diversité dans ce clade ce qui était inattendu pour des organismes osmotrophes vivant dans des habitats oligotrophes. De plus, la plupart des phylotypes trouvés ne sont pour le moment pas décrits morphologiquement ni génétiquement, ce qui suggère l’existence d’organismes hautement spécialisés.
    Nous avons également étudié la diversité des micro-eucaryotes vivant dans des Sphaignes situées à différentes altitudes dans trois zones climatiques différentes : tempérée (Suisse-France-Italie), subtropicale (Japon) et tropicale (Costa Rica). Nos résultats suggèrent que 25% des phylotypes étaient communs dans ces trois zones. Nous avons également trouvé une corrélation significativement négative entre la quantité de phylotypes liés aux organismes mixotrophes et des températures élevées. Cela suggère que la mixotrophie est désavantageuse dans un climat chaud. Enfin, nous avons étudié la répartition spatiale d’une espèce emblématique d’amibe à thèque trouvé dans les tourbières de l’hémisphère nord: Hyalosphenia papilio. Un total de 13 lignées ont été trouvées, dont neuf présentent des distributions restreintes et quatre sont bien réparties dans tout le domaine holarctique. Nous avons montré, sur la base de reconstructions phylogénétiques et d’une reconstitution des caractères ancestraux, que l’origine de H. papilio se situe probablement sur la côte ouest de l’Amérique du Nord.
    En résumé, ma thèse démontre que l’environnement « sphagnosphère » accueille une diversité élevée et unique de micro-eucaryotes. Cette diversité est influencée par des variables environnementales physicochimiques à l’échelle locale mais également par le climat et la distance géographique à l’échelle mondiale. Nous avons identifié et quantifié les principales variables abiotiques locales (à savoir la microtopographie et la teneur en azote) qui influencent fortement les communautés au sein d’une même zone climatique. Ces variables ont exercé un fort effet de filtre environnemental, qui semble être un processus fondamental dans la mise en place des communautés. De plus, à l’échelle mondiale, nous avons démontré que la température était le principal paramètre influençant la composition de la communauté, et notamment l’abondance mixotrophique. Aux deux échelles, la composition des communautés, et donc les interactions biotiques (et probablement le fonctionnement des écosystèmes), changent radicalement., Despite the fact that free-living microeukaryotes compose the major part of Earth’s biodiversity and play numerous essential roles in ecosystems, knowledge on their true diversity, ecology and their global patterns of distribution remain limited. In this sense, the objectives of this thesis are 1) to increase the knowledge on the diversity of microeukaryotes 2) characterize the ecological preferences and determine which are the main variables that influence community composition, and finally 3) to understand the rules that shape the communities at both local and global scales. To meet these objectives a specific component of the earth surface was selected: the “Sphagnosphere” i.e. the interstitial water directly influenced by Sphagnum mosses. This understudied but unique microenvironment is characterized by low nutrient contents, low pH, and high amounts of organic acids produced by the mosses. It is also very stable over time.
    We first explored the diversity of two groups of protists in Sphagnum peatlands. The first group was genus Nebela (Arcellinida, Hyalospheniidae), a common testate amoeba taxon in acidic soils. We formally described the most abundant one and named it Nebela gimllii due to the small and stout shells. The different community profiles revealed that species are not randomly distributed among microhabitats in peatlands. Instead, we observed a strong phylogenetic clustering in nitrogen-poor areas suggesting that little amounts of nitrogen exerted strong environmental filtering. We also surveyed the molecular diversity of Oomycota, a clade of fungi- like stramenopiles which enclose many animal, fungi and plant parasites, as well as saprotrophic species. We revealed a high diversity, which was unexpected for osmotrophic organisms in nutrient-poor habitats unless most are parasitic. Moreover, most phylotypes found were not recorded in previous studies, which suggest the existence of highly specialized organisms.
    We also surveyed the diversity of microbial eukaryotes along altitudinal gradients in three different climatic zones, temperate (western Alps), subtropical (Japan) and tropical (Costa Rica). We showed that 25 percent of phylotypes were shared in the three climatic zones. We found also a significant negative correlation between the proportion of phylotypes related to mixotrophic organisms and temperature. This, in line with other lines of evidence in the literature corroborates the idea that mixotrophy is disadvantageous under warm climates. Finally, we studied the spatial distribution of an emblematic morphospecies of testate amoeba found in the northern hemisphere peatlands: Hyalosphenia papilio. A total of 13 lineages were found, from which nine showed narrowly restricted distributions, and four were well distributed across the Holarctic realm. We showed, based on phylogenetic analyses and ancestral character reconstructions that H. papilio most probably appeared somewhere in the West Coast of North America.
    In summary, my PhD revealed that the Sphagnosphere environment hosts high and unique diversity. This diversity is driven by physicochemical factors at the local scale and by climate and geographical distance at the global scale. We identified and quantified the main local abiotic variables, amongst which micro-topography and nitrogen content appeared to be the most significant in shaping micro-eukaryotic diversity within the same climate zone. These variables exerted strong environmental filtering, which appeared to be fundamental process of community assembly. On the other hand, at a global scale, we demonstrated that temperature was the factor that best explain community composition, and notably the abundance of mixotrophs (and hence a different functioning). At both scales, community composition, and therefore biotic interactions (and most probably ecosystem functioning) change drastically.
  • Publication
    Accès libre
    A multi-proxy, high-resolution record of peatland development and its drivers during the last millennium from the subalpine Swiss Alps
    (2011)
    van der Knaap, W.O.
    ;
    Mariusz Lamentowicz
    ;
    van Leeuwen, J.F.N
    ;
    Hangartner, S.
    ;
    Leuenberger, M.
    ;
    Dmitri Mauquoy
    ;
    Golslar, T.
    ;
    Mitchell, Edward 
    ;
    Kamenikg, C.
    We present a record of peatland development during the last 1000 years from Mauntschas mire in the eastern Swiss Alps (Upper Engadine valley; 1818 m a.s.l.) inferred from testate amoebae (pH and depth to the water table (DWT) reconstructions), stable oxygen isotopes in Sphagnum (δ18O; proxy for water vapour pressure) and carbon isotopes in Sphagnum (δ13C; proxy for mire surface wetness), peat accumulation rates, charcoal (indicating local burning), pollen and spores (proxies for human impact), and plant macrofossils (reflecting local vegetation and trophic state). Past human impact on the local mire conditions was strong but fluctuating during AD 1000–1570 (±50 yr; depth–age model based on 29 14C AMS dates) with local irrigation of nutrient-enriched water and grazing. Human impact was minor AD 1570–1830 (±30 yr) with partial recovery of the local mire vegetation, and it was absent AD 1830 (±30 yr)–present when hummock formation took place. Correlations among DWT, pH, δ13C, and δ18O, carried out both with the raw data and with linear trends removed, suggest that the factors driving peatland development changed over time, since only testate amoeba-based pH and DWT co-varied during all the three aforementioned periods. δ18O correlates with δ13C only in the period AD 1830–present and with DWT only during AD 1570–1830, δ13C correlates with DWT only during AD 1000–1570. Part of this apparent instability among the four time series might be attributed to shifts in the local mire conditions which potentially formed very different (non-analogue) habitats. Lack of analogues, caused, for example, by pre-industrial human impact, might have introduced artefacts in the reconstructions, since those habitats are not well represented in some proxy transfer functions. Human impact was probably the main factor for peatland development, distorting most of the climate signals.
  • Publication
    Accès libre
    Testate amoebae analysis in ecological and paleoecological studies of wetlands: past, present and future
    (2008)
    Mitchell, Edward 
    ;
    Charman, Daniel J.
    ;
    Warner, Barry G.
    Testate amoebae are an abundant and diverse polyphyletic group of shelled protozoa living in aquatic to moist habitats ranging from estuaries to lakes, rivers, wetlands, soils, litter, and moss habitats. Owing to the preservation of shells in sediments, testate amoebae are useful proxy indicators complementary to long-established indicators such as pollen and spores or macrofossils. Their primary use to date has been for inferring past moisture conditions and climate in ombrotrophic peatlands and, to a lesser extent, to infer pH in peatlands and the trophic or nutrient status of lakes. Recent research on these organisms suggests other possible uses in paleoecology and ecology such as sea-level reconstruction in estuarine environments, as indicators of soil or air pollution, and monitoring recovery of peatland. We review the past and present use of testate amoebae, the challenges in current research, and provide some ideas on future research directions.
  • Publication
    Accès libre
    Effect of taxonomic resolution on ecological and palaeoecological inference: a test using testate amoeba water table depth transfer functions
    Mitchell, Edward 
    ;
    Lamentowicz, Mariusz
    ;
    Payne, Richard J
    ;
    Mazei, Yuri
    Sound taxonomy is a major requirement for quantitative environmental reconstruction using biological data. Transfer function performance should theoretically be expected to decrease with reduced taxonomic resolution. However for many groups of organisms taxonomy is imperfect and species level identification not always possible. We conducted numerical experiments on five testate amoeba water table (DWT) transfer function data sets. We sequentially reduced the number of taxonomic groups by successively merging morphologically similar species and removing inconspicuous species. We then assessed how these changes affected model performance and palaeoenvironmental reconstruction using two fossil data sets. Model performance decreased with decreasing taxonomic resolution, but this had only limited effects on patterns of inferred DWT, at least to detect major dry/wet shifts. Higher-resolution taxonomy may however still be useful to detect more subtle changes, or for reconstructed shifts to be significant.
  • Publication
    Accès libre
    Eight species in the Nebela collaris complex: Nebela gimlii (Arcellinida,Hyalospheniidae), a new species described from a Swiss raised bog
    Singer, David 
    ;
    Kosakyan, Anush 
    ;
    Pillonel, Amandine 
    ;
    Mitchell, Edward 
    ;
    Lara, Enrique 
    We describe here a new species of sphagnicolous testate amoeba found abundantly in the forested part of the Le Cachot peatland (Jura Mountains, Neuchâtel, Switzerland) based on microscopical observations (LM, SEM). The new species, called Nebela gimlii was placed in a phylogenetic tree based on mitochondrial cytochrome oxidase sequences (COI), and branched robustly within the N. collaris complex next to the morphologically similar N. guttata and N. tincta. It is however genetically clearly distinct from these two species, and differs morphologically from them by its smaller size and stouter shape of the shell. This new species completes the phylogeny of the Nebela collaris species complex, with now eight species described, mostly from peatlands and acidic forest litter, and further demonstrates the existence of an unknown diversity within testate amoebae. Improving the taxonomy of testate amoebae in peatlands and clarifying the ecology of newly discovered species should make these organisms even more valuable as bioindicator and for palaeoecological reconstruction.
  • Publication
    Accès libre
    Towards a Holarctic synthesis of peatland testate amoeba ecology: Development of a new continental-scale palaeohydrological transfer function for North America and comparison to European data
    Amesbury, Matthew J
    ;
    Booth, Robert K
    ;
    Roland, Thomas P
    ;
    Bunbury, Joan
    ;
    Clifford, Michael J
    ;
    Charman, Dan J
    ;
    Elliot, Suzanne
    ;
    Finkelstein, Sarah
    ;
    Garneau, Michelle
    ;
    Hughes, Paul D.M
    ;
    Lamarre, Alexandre
    ;
    Loisel, Julie
    ;
    Mackay, Helen
    ;
    Magnan, Gabriel
    ;
    Markel, Erin R
    ;
    Mitchell, Edward 
    ;
    Payne, Richard J
    ;
    Pelletier, Nicolas
    ;
    Roe, Helen
    ;
    Sullivan, Maura E
    ;
    Swindles, Graeme T
    ;
    Talbot, Julie
    ;
    van Bellen, Simon
    ;
    Warner, Barry G
    Fossil testate amoeba assemblages have been used to reconstruct peatland palaeohydrology for more than two decades. While transfer function training sets are typically of local-to regional-scale in extent, combining those data to cover broad ecohydrological gradients, from the regional-to continental- and hemispheric-scales, is useful to assess if ecological optima of species vary geographically and therefore may have also varied over time. Continental-scale transfer functions can also maximise modern analogue quality without losing reconstructive skill, providing the opportunity to contextualise understanding of purely statistical outputs with greater insight into the biogeography of organisms. Here, we compiled, at moderate taxonomic resolution, a dataset of nearly 2000 modern surface peatland testate amoeba samples from 137 peatlands throughout North America. We developed transfer functions using four model types, tested them statistically and applied them to independent palaeoenvironmental data. By subdividing the dataset into eco-regions, we examined biogeographical patterns of hydrological optima and species distribution across North America. We combined our new dataset with data from Europe to create a combined transfer function. The performance of our North-American transfer function was equivalent to published models and reconstructions were comparable to those developed using regional training sets. The new model can therefore be used as an effective tool to reconstruct peatland palaeohydrology throughout the North American continent. Some eco-regions exhibited lower taxonomic diversity and some key indicator taxa had restricted ranges. However, these patterns occurred against a background of general cosmopolitanism, at the moderate taxonomic resolution used. Likely biogeographical patterns at higher taxonomic resolution therefore do not affect transfer function performance. Output from the combined North American and European model suggested that any geographical limit of scale beyond which further compilation of peatland testate amoeba data would not be valid has not yet been reached, therefore advocating the potential for a Holarctic synthesis of peatland testate amoeba data. Extending data synthesis to the tropics and the Southern Hemisphere would be more challenging due to higher regional endemism in those areas.